Telecommunications products often perform mixed signal processing, which is simultaneous processing of both analog and digital signals. For example, a digital cordless telephone handset receives an analog speech signal via a microphone, converts the speech signal into a digital speech signal, compresses the digital speech signal, modulates the compressed signal at a radio frequency (RF), and transmits the modulated RF signal through an antenna. The transmitted RF signal is received by a nearby base station, converted back to an analog signal, and ultimately relayed to the destination telephone. When a similar signal is received from the destination telephone, the telephone signal undergoes the same process; the base station then transmits a corresponding digital RF signal. The RF signal is received at the handset via the antenna, demodulated, decompressed, and converted into an analog speech signal which drives a speaker in the handset. Thus, both analog and digital functions are necessary in the operation of a digital cordless telephone handset.
However, analog and digital circuitry used in mixed signal processing applications have different power supply requirements. For example, digital circuits are commonly manufactured with complementary metal-oxide-semiconductor (CMOS) technology. CMOS has the advantage that power consumption is relatively low compared to other technologies. Digital CMOS circuits operate with a wide range of power supply voltages, for example from the conventional +5.0 volts down to +3.0 volts and below. At higher power supply voltages, CMOS circuits are faster but have other problems. They consume more power than at lower power supply voltages. They also have reliability problems above certain voltages. Since digital CMOS logic provides logic levels which are essentially full supply, the higher supply voltage is driven onto gates of CMOS transistors. If the voltage at the gate of a CMOS transistor is too large, gate rupture or gate oxide degradation may result. Hence, power supply voltages must be limited to prevent the reliability problems. On the other hand, analog circuitry often requires a higher minimum power supply voltage than digital circuitry. For example, many amplifiers have inherent power supply headroom limitations which cause the amplifiers to distort the output signal if the power supply voltage compresses too much.
To complicate matters, digital cordless telephone handsets and many other mixed signal processing environments require battery operation. The battery may be, for example, one or more rechargeable nickel-cadmium (nicad) batteries or their equivalent. However, battery voltages vary widely between recharges. For example, a single nicad battery, type AA, may reach a voltage of 1.7 to 1.8 volts immediately after a recharge, but drop to a voltage of 0.9 to 1.0 volts before going dead. Thus, three type AA nicad batteries provide a voltage range from about 5.4 volts to about 2.7 volts between recharges.
Known mixed signal processing systems are not effective in operating within the differing power supply requirements. For example, a power supply voltage provided by a battery may be input to a charge pump to increase the voltage provided to some of the internal circuitry. In this case, the digital circuitry receives the battery voltage, and the analog circuitry receives the higher, charge-pumped voltage. Obviously, such designs require a narrow tolerance of battery voltages so that both the digital and analog circuitry are able to function properly. Thus, because of their variability in voltage, nicad and other types of batteries are poorly suited for these types of systems. If the battery power supply voltage is near the top of its range, then the charge-pumped voltage is very large and significantly increases power consumption and reduces reliability. As power consumption increases, battery life shortens.